Support const-bound counter loop generation for 'downTo'

compiler/testData/codegen/box/ranges/forInDownTo/forIntInDownToWithNonConstBounds.kt

@@ -0,0 +1,16 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun low() = 4
+fun high() = 1
+
+fun box(): String {
+    var sum = 0
+    for (i in low() downTo high()) {
+        sum = sum * 10 + i
+    }
+    assertEquals(4321, sum)
+
+    return "OK"
+}